docs: refresh img/ summary charts; add img index; cite reproducible stats

- Regenerated the three README charts from data/synthetic_1.2.6 via
  scripts/univariate_analysis.py --img (replaces the Feb 2026 ad-hoc versions)
- img/README.md: documents img/ as the published-docs chart directory
  (stable README link targets), with provenance and the regeneration command
- README: skew/mean narrative updated to the reproducible script output
  (mean 5.44, median 5.48; Fisher skew -0.03) with pointer to
  univariate_summary.txt, keeping the historical ODS reference

analysis/img/README.md

@@ -0,0 +1,24 @@
+# img/
+
+Published summary charts for the README's analysis narrative (stable link
+targets, dataset-independent). These are *copies* — each run's full set of
+plots lives in `data/<run>/analysis/plots/`.
+
+| File | Chart | Source run |
+|---|---|---|
+| `protocol_averages.png` | Per-protocol average gradient values, ascending | synthetic_1.2.6 |
+| `gradient_averages.png` | Per-gradient averages, gaps separate the three sets | synthetic_1.2.6 |
+| `averages_histogram.png` | Distribution of protocol averages | synthetic_1.2.6 |
+
+Regenerate these from any run:
+
+```bash
+python3 scripts/univariate_analysis.py data/synthetic_1.2.6/readings.csv --img
+```
+
+(`--img` with no argument publishes to this directory; `--img <dir>` targets
+another one. The corresponding summary statistics are written to the run's
+`analysis/reports/univariate_summary.txt`.)
+
+Last refreshed: 2026-10-02, from `data/synthetic_1.2.6/` via
+`scripts/univariate_analysis.py` (`bicorder v1.2.6`).